House Mouse.The most common household rodent is the house mouse, which resembles the roof rat in that they both have large ears, pointed muzzles and slender bodies. The house mouse is a small, slender, dusky-gray rodent with a slightly pointed nose; small, black, protruding eyes; and large, scantily haired ears. The adult mouse can be distinguished from a young roof rat because the head and feet of the mouse are distinctly smaller in proportion to its body size. Adults weigh ½ to 3/4 ounce and are 2½ to 3½ inches long in head-and-body length. The hairless tail is 3 to 4 inches long. The feces are 1/8 inch to ¼ inch long and are rod-shaped.

House mice are considered among the most troublesome and economically important rodents in the U.S. Although house mice are commonly found living in man-made structures, they are also well adapted to living outdoors, being common inhabitants of grassy fields and cultivated grain crops. These wild populations often move into buildings when weather becomes severe. House mice have poor vision and are color-blind. Mice use their sense of smell to locate food items and recognize other individual mice. House mice have acute hearing and readily respond to unusual noises as a means of detecting and escaping danger. However, they become accustomed to repetitive, ordinary noises, and as a result, their activities may be more visible than those of rats. An important sensory factor is touch. Mice use the long, sensitive whiskers on the nose and above the eyes as tactile sensors. The whiskers and guard hairs enable the mice to travel easily in the dark along runways close to walls.

House mice feed on a wide range of foods, although they seem to prefer cereals over other items. In particular, most mice favor the germ of grains. As supplemental diet items, mice often show preference for foods high in fat and protein, such as lard, butter, nuts and dried meats. House mice are sporadic feeders, nibbling bits of food in various locations throughout their range. Peak feeding periods are at dusk and just about dawn, but, because of their small size, mice must feed several times during a 24-hour period and thus are active day and night. They normally range 10 to 30 feet from the nest, which is often lined with soft materials such as cotton or paper and may be built in walls, cabinets, upholstered furniture or other convenient spaces. Urine and droppings mark the trail for others. Unlike some rats, mice are poor swimmers.

Rats. Two types of urban
rats are broadly distributed, the Norway rat and the roof rat. The
Norway rat (synonymous with brown, dump, barn, sewer, gray or wharf rat)
is a burrowing rodent. The Norway rat has a blunt muzzle, small eyes
and short, close-set ears. Its fur is coarse and usually brownish- or
reddish-gray, with whitish-gray hair on the belly. Its nearly naked,
scaly tail is dark on the top and light on the underside and is shorter
(6 to 8½ inches) than the combined length of the head and body (7 to 10
inches). Adults weigh 12 to 18 ounces.

The feces are capsule-shaped and about 3/4 inch long. Norway rats can be found in warehouses, farm buildings, houses, sewers, rubbish dumps, woodpiles and building foundations. They are good climbers and can reach a distance of 13 inches while standing on the ground and jump 24 inches vertically. The Norway rat has relatively poor vision but keen senses of smell, touch, taste and hearing. The sense of touch is served by long whiskers on the snout. The home range is often 100 to 150 feet. Norway rats are mainly nocturnal, but they may be active in undisturbed places during the day. They feed on virtually anything edible.

The roof rat (black or ship rat) is somewhat smaller and is a more agile climber. It has several color phases, a slender body, prominent ears and large eyes. Roof rats have large, membranous ears and sharply pointed muzzles. The unicolored, nearly hairless tail (7½ to 10 inches) is usually longer than the head and body combined (6½ to 8 inches). The adult weighs 8 to 12 ounces, and the feces differ from those of the Norway rat in that they are about ½ inch long and spindle-shaped. Serious pest populations of roof rats are confined along the southern and western coastal areas of the country.Normally rats and mice are nocturnal, so recognition of various signs is necessary in determining population levels. Some of these signs are burrows, gnawing activity, fecal droppings, runways, rub marks, tracks and carcasses. Reproduction, mortality and movement into and out of an area determine the potential size of rodent populations, whereas physical environment, food, shelter, water, predation, parasitism and competition control the actual population size.Sylvan rodents. Deer (white-footed) mice and wood rats may enter buildings when they are seeking harborage or food. Deer mice are about the same size or slightly larger than house mice but can be differentiated from house mice by a distinct, bicolored tail (upper portion brown-gray, lower half white). They characteristically have small ears and eyes and a relatively short tail. The deer mouse is the most common reservoir of Sin Nombre virus, the causative agent of hantavirus pulmonary syndrome, but other small rodents also may carry the disease. Infected rodents shed the virus in their saliva, urine and droppings. The virus becomes airborne when fresh rodent urine, droppings or nesting materials get stirred up. Most people acquire hantavirus by inhalation of the airborne virus. Hantavirus pulmonary syndrome is a severe respiratory illness that results in death for about 40 percent of its victims. Use extreme caution or avoid activities associated with exposure to mouse or small-animal droppings. When conducting rodent control or cleanup activities, disinfect droppings, nesting materials, rodent carcasses and traps. In extremely infested environments wear respiratory protection.